Apparatus and methods for uniform frequency sample clocking

ABSTRACT

The invention provides systems and methods for producing an OCT image from a swept laser source. In certain embodiments, methods of the invention involve producing light from a swept laser source, splitting the light into a first and a second portion, in which the first portion is sent to an OCT interferometer to produce an OCT signal and the second portion is sent to a uniform frequency sample clock, processing the second portion into an external clock signal that is accepted by a digitizer, sending the OCT signal and the clock signal to the digitizer, and generating an OCT image from a combination of the OCT signal and the clock signal.

CROSS-RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/172,980, filed Jul. 14, 2008, which claims priority to U.S.Provisional Application Ser. No. 60/949,467, filed Jul. 12, 2007, thecontents of which are incorporated by reference herein in theirentireties.

BACKGROUND

The invention is generally related to laser systems, and moreparticularly related to Optical Coherence Tomography systems andmethods.

Optical Coherence Tomography (“OCT”) generates cross sectional imagingof tissues with micro-scale resolution by measuring the echo time delayof backscattered or backreflected light. Fourier Domain OCT (“FD-OCT”)can obtain a high sensitivity and imaging speed by acquiring the opticalspectrum of light backscattered from a sample interfered with light froma single stationary reference reflector using an interferometer.Swept-Source OCT (“SS-OCT”), time-encodes the wavenumber (or opticalfrequency) by rapidly tuning a narrowband light source over a broadoptical bandwidth.

The high speed tunable laser sources for SS-OCT exhibit a nonlinear ornon-uniform wavenumber vs. time [k(t)] characteristic. As such, SS-OCTinterferograms sampled uniformly in time [S(t), e.g., using an internaldigitizer clock] must be remapped to S(k) before Fourier transforminginto the path length (z) domain used to generate the OCT image. Thissoftware remapping operation is undesirable due to decreased phasesensitivity (i.e., amplitude and phase errors introduced in theremapping procedure), image artifacts, computational inefficiency, andthe bandwidth needed to record a remapping array [k(t)] for each OCTinterferogram [S(t)].

Additionally, when operating at fast frequency sweep speeds, the laserfrequency varies nonlinearly rather than linearly with time in SS-OCT.Therefore, an accurate and reliable recalibration of the interferenceoutput to equidistant spacing in wavenumber (k) or optical frequency (v)is necessary. Also, nonlinearity may arise from the specifics of thetunable element(s) in the source, which might be the type of tunablefilter (e.g., mechanical rotating mirror in combination with a grating,fabry perot, etc.), the operation of the tunable filter or time varyingoperation (drift) of the tunable filter. Analog-Digital (“A/D”)converter cards that operate at lower frequencies, i.e. low-speeddigitizers, are not practical for the high-speed OCT systems. Moreover,most high speed A/D converter cards have a timing jitter of thegenerated clocking signal, which can degrade images recorded usingSS-OCT. Most high-speed digitizers are very selective about the timingparameters of their external clock, i.e. duty cycle, pulse shape,amplitude, jitter, etc. Thus, the external clocking is difficult withoutsome way of processing, whether analog or digital.

High-speed digitizers operating with an internal sample clock andswept-source lasers used for OCT must have a calibration performedbetween the non-linear sweep of the laser and the constant samplingfrequency of the digitizer. This may be accomplished using softwarepost-processing interpolation method of a linearly-sampled (in time)calibration k-space wavemeter signal or a non-uniform Fourier transform.Despite these approaches, obtaining a uniformly-sampled signal inwavenumber (k) or optical frequency (ν) improves OCT image quality anddisplay rate. Unfortunately, today's high-speed digitizers havestringent requirements on the continuity of the amplitude and frequencyof the external sample clock input, and disruptions in the wavemetersignal for example at times when the laser is between successive sweepscan disable the digitizer's internal sampling circuitry. The embodimentsdescribed herein solve these problems, as well as others.

SUMMARY OF THE INVENTION

The method and a system for uniform frequency sample clocking directlysamples the OCT signal with a temporally-non-linear sampling clockderived from a k-space wavemeter on the external sample clock input portof a digitizer.

The foregoing and other features and advantages are defined by theappended claims. The following detailed description of exemplaryembodiments, read in conjunction with the accompanying drawings ismerely illustrative rather than limiting, the scope being defined by theappended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing description of the figures is provided for a more completeunderstanding of the drawings. It should be understood, however, thatthe embodiments are not limited to the precise arrangements andconfigurations shown.

FIG. 1 is a schematic of the Uniform Frequency Sample Clock Pathways forthe external clocking of a swept laser source.

FIG. 2 is a schematic of one embodiment of the Uniform Frequency SampleClock coupled with an OCT Interferometer.

FIG. 3 is a schematic of one embodiment of Pathway 1.

FIG. 4A is a schematic of one embodiment of Pathway 2; FIG. 4B is aschematic of the trigger generator; and FIG. 4C is a schematic of theauxiliary wavemeter.

FIG. 5 is a schematic of the digitizer with the arbitrary waveformgenerator.

FIGS. 6A and 6B are cross-sectional view of schematics for alternativeembodiments of the auxiliary wavemeter.

FIG. 7 is a schematic of one embodiment of Pathway 2.

FIG. 8 is a schematic of one embodiment of the OCT interferometer.

FIG. 9 is a schematic diagram of the circuit for direct external sampleclocking of swept-source OCT using an optical wavemeter.

FIG. 10A is a schematic of a Calibration Gas Cell and the auxiliarywavemeter coupled from the swept source laser; FIG. 10B is a schematicof the optical wavemeter where light making a single-pass 316 and atriple-pass 318 between partial reflectors interferes and produces aperiodic signal uniformly spaced in wavenumber (k); and FIG. 10C is aschematic of the calibration gas cell.

FIG. 11 is a schematic of one embodiment of Pathway 6.

FIG. 12 is a graph schematic of one embodiment of external clockcircuitry.

FIG. 13 is a schematic of common-path OCT interferometer inphase-sensitive Fourier Domain OCT.

FIG. 14 is a block diagram of phase sensitive Fourier-domain OCTinstrument with the Real-Time Imaging Clocking system.

FIG. 15 is an OCT point spread functions vs. depth for an internallyclocked/remapped scheme (440) and the externally clocked scheme 442

FIG. 16 is a schematic of one embodiment of the phase sensitive OCTinterferometer configuration.

FIG. 17A-17C are graphs of the change in thickness in the piezofilm inresponse to application of a periodic voltage at increasing frequency(17A: 500 Hz, 17B: 1000 H, 17C: 2000 Hz).

FIG. 18 is a schematic of the demultiplexer coupling multiple clocksignals from various pathways.

DETAILED DESCRIPTION OF THE INVENTION

The methods, apparatuses, and systems can be understood more readily byreference to the following detailed description of the methods,apparatuses, and systems, and the accompanying figures.

Generally speaking, a Uniform Frequency Sample Clocking 10 systems andmethods for a swept laser source 12 are generally shown in FIG. 1. TheUniform Frequency Sample Clocking 10 comprises at least one Pathway,where some embodiments of the Pathways are generally shown as linearrows in FIG. 1. The line arrows represent electronic or opticalcoupling elements, such as wires, fibers, and the like. In oneembodiment, Uniform Frequency Sample Clocking 10 includes Pathway 1comprising characterizing 26 the swept laser source 12, creating adigital representation of the waveform based from the characterizationdata 26, and generating a clock signal 30 using a waveform generator 14(i.e. a Digital-Analog (“D/A”) converter) to output the clock signal 30to a digitizer external clock 32. The Uniform Frequency Sample Clocking10 may include Pathway 2 comprising coupling the swept laser source 12to an auxiliary wavemeter 16, digitally processing the auxiliarywavemeter 16 output with an Analog-Digital (“A/D”) digitizer 18, andprocessing the digitizer's 18 output on the D/A converter 14 to generatethe clock signal 30 outputted to the digitizer external clock 32. TheUniform Frequency Sample Clocking 10 may include Pathway 3 comprisingcoupling the swept laser source 12 to the auxiliary wavemeter 16 andprocessing auxiliary wavemeter 16 output using an analog processor 20 togenerate the clock signal 30. The Uniform Frequency Sample Clocking 10may include Pathway 4 comprises coupling the swept laser source 12 tothe auxiliary wavemeter 16, processing the auxiliary wavemeter 16 outputwith the analog processor 20, digitizing the analog processor's 20output with the digitizer 18, digitally processing the auxiliarywavemeter 16 output with the D/A converter 19 to generate the clocksignal 30. The Uniform Frequency Sample Clocking 10 may include Pathway5 comprising coupling the swept laser source 12 to the auxiliarywavemeter 16 to directly generate a uniform-frequency sample clocksignal with no pre-processing. The Uniform Frequency Sample Clocking 10may include Pathway 6 coupling the swept laser source 12 to a UniformFrequency Sample Clock Generator 22 outputting to a digitizer 24 togenerate the clock signal 30. The Uniform Frequency Sample Clocking 10systems and Pathways provide for external clocking of the swept lasersource 12 and can provide a different clocking signal throughindependent Pathways, in combination and in any particular order, togenerate the clock signal, process the clock signal, and transmit theclock signal to the digitizer for uniform sampling of detected light inthe wavenumber domain. For each acquisition channel, one clock signalmay be active at a given time, which may be switched between differentclock signals in any particular combination or order. Alternatively, theUniform Frequency Sample Clocking 10 Pathways may be combined with eachother, in any sequence of combinations. More particularly, the UniformFrequency Sample Clock Pathways 10 provide external clocking of detectedlight first emitted from the swept laser source for OCT systems. Theterm “Uniform Frequency Sample Clocking” and “linear sampling in thewavenumber domain” are equivalent terms, as used in the specification.The term “external clock signal” is specific to the type of signalapplied to the external clock signal input or the clock signal input ofthe digitizer external clock 32. The term “clock signal” is the signalas applied to the AD converter card.

The swept laser source 12 includes emitted light with a mean frequencyof the output spectrum that varies over time. The term “swept lasersource” is synonymous with a “tunable laser source”, i.e. tuning a lasersource over a period of time at a certain frequency. The mean opticalfrequency of light emitted from the swept source may change continuouslyover time at a tuning speed that is greater than 100 terahertz permillisecond and repeatedly with a repetition period. range of sweepspeeds for the table laser source—specifying a range of sweep speeds(e.g., 10,000-10,000,000 Sweeps/sec). The swept laser source 12 may beany tunable laser source that rapidly tunes a narrowband light emissionthrough a broad optical bandwidth. The tuning range of the swept sourcemay have a tuning range with a center wavelength between approximately500 nanometers and 2000 nm, a tuning width of approximately greater than1% of the center wavelength, and an instantaneous line width of lessthan approximately 10% of the tuning range. Alternatively, more than oneoptical source may be combined to produce the swept laser source, or acontinuously swept multi-wavelength laser emitting several opticalfrequencies or wavelengths simultaneously. While tunable lasers andswept-source lasers are included as the swept laser source, FourierDomain Mode Locking (“FDML”) lasers may be included as the laser source.In FDML, the spectrum, rather than the amplitude of the field, ismodulated. A dynamic spectral window function (wavelength window whichchanges in time), rather than a temporal one (time window with nowavelength dependence), is applied. As a result, the laser generates asequence of narrowband optical frequency sweeps at the cavity repetitionrate or a harmonic thereof. Multiple tunable wavelength sources mayincluded with, where each tunable wavelength source has a receiver, soeach tunable wavelength source is coupled with a detector. The compositeof all the tunable wavelength laser sources and detectors can act asvery large bandwidth laser source. This frequency-swept output can alsobe thought of as a sequence of highly chirped, long pulses, with a fixedphase relationship between successive frequency sweeps.

In one embodiment, the swept laser source 12 provides the swept opticaloutput to an OCT interferometer 40 and the Uniform Frequency SampleClock 10, as shown in FIG. 2. Light emitted from the swept laser source12 is split between the OCT interferometer 40 and the uniform frequencysample clock 10. The swept laser source 12 may be split in any desiredratio, including, but not limited to 95/5, 90/10, 85/15, 80/20, etc. tothe OCT interferometer 40 and the Uniform Frequency Sample Clock 10,respectively. The Uniform Frequency Sample Clock 10 and the OCTinterferometer 40 are then coupled to the digitizer 32 and then to acomputer for processing and imaging.

The OCT interferometer 40 splits the light emitted from the swept sourceto a reference surface and a sample arm, which recombines at the outputof the interferometer. The OCT interferometer may take any of thevariety of configurations known in the art, including, but not limitedto, a Michelson interferometer, a Mach-Zehnder Interferometer, and/or acommon path interferometer, etc. The Uniform Frequency Sampling clockgenerator 22 receives light from the swept source or the previouslycharacterized swept source data and outputs to the digitizer to providelinear sampling in the wavenumber (k) or optical frequency (ν) domain,allowing direct Fourier transformation into the pathlength (z) domainfor real time OCT imaging. The real-time OCT imaging comprises (1)Uniform Frequency Sampling; and (2) direct Fourier transformation offringe data for real-time OCT imaging. Direct Fourier transformationrequires a digital processing element that does the Fourier transform ofthe Uniform Frequency Sampled OCT fringe data or the “OCT signal data”.Uniform Frequency Sampled OCT fringe data or the “OCT signal data” areused synonymously throughout. Generally speaking, the Uniform FrequencySample clock 10 provides for linear sampling in the wavenumber domain,digitizing the OCT fringe data in the wavenumber domain for real-timeOCT imaging, and combinations thereof. FIG. 15 compares axial pointspread functions and OCT images generated with uniform time sampling vs.the uniform frequency sample clocking approach 10 using the Pathwaysdiscussed below.

Pathway 1: Characterizing the Swept Laser Source

In one embodiment, the Uniform Frequency Sample Clock 10 includesPathway 1 comprising a step of characterizing light emitted by the sweptlaser source 12, creating a digital representation of the waveform basedon the characterization data 26, and repeatedly outputting thecharacterization data 26 for each subsequent optical trigger that occursas the laser is sweeping, as shown as Pathway 1 in FIG. 1. Data forcharacterizing light emitted from the swept laser source(“characterization data”) is generated using a high-speed D/A converter,i.e. the waveform generator 14, which is then coupled to the digitizer's32 external clock input port. The D/A converter 14 outputs the generatedUniform Frequency Sample Clock signal for each laser sweep, triggered byan electrical synchronization pulse or an optical trigger 54 derivedfrom the swept-source laser output. There are several ways to generatethe optical trigger 54. In one embodiment, the optical trigger 54 isgenerated from an optical trigger generator 60, discussed below. Inanother embodiment, the optical trigger 54 is derived from an opticaltuning element in the swept laser source, as the signal may be generatedactively or passively. When light is emitted from the swept laser sourceand interacts with an optical tuning element at the output of the sweptlaser source, the optical trigger signal is provided. The optical tuningelement may be static and does not necessarily need to be actively tunedto function in the role providing the optical trigger signal. Anotherembodiment to generate the optical trigger 54 is to sample the lightwith an optical frequency selective element, i.e. a gratingspectrometer, interference filter, Fabry-Perot filter, and the like, orcombination there-of, and a photo-detector coupled to an A/D converterto provide the optical trigger. So there may be two different opticaltuning elements, one within the laser source that functions to tune thelaser and one that can be placed at the output of the tuning lasersource, which can be used to provide a trigger signal. Combinations ofthese optical tuning element embodiments can be employed to generate anoptimum optical trigger 54.

In one embodiment, the D/A converter 14 may comprise an arbitrarywaveform generator 50, as shown in FIG. 3. The arbitrary waveformgenerator 50 (CompuGen, Lockport, Ill.) provides aperiodic or periodicanalog waveforms 52 as their output and generates a pre-programmedwaveform every time a trigger event occurs. The pre-programmed waveformis stored in the on-board memory of the arbitrary waveform generator.Arbitrary waveforms 52 are generated by creating a digitalrepresentation of the waveform based on the characterization data 26 oflight emitted from the swept laser source in the memory of the arbitrarywaveform generator. The digital representation pattern is converted intoan analog signal using a high-speed Digital-to-Analog converter andconditioning amplifiers (buffers and attenuators) within the arbitrarywaveform generator 50. The external clock signal 34 is derived from thecharacterization data of the swept source during a start-up calibrationstep, and then repeatedly outputted by the arbitrary waveform generatorfor each subsequent optical trigger 54 signal that occurs as the laseris sweeping. Alternatively, the external clock signal 34 fromcharacterization data 26 can also be completed periodically according tosome schedule programmed by a computer software 58, or may be performedin response to some event such as a parameter (or combination ofparameters) of the source changing (e.g., temperature). The uniformfrequency sample clock by the characterization of the swept laser sourcedata 26 allows acquisition (analog to digital conversion) of OCTinterferometer 40 data directly in wavenumber (k) space.

As shown in FIG. 1, characterizing the swept laser source data 26 mayinclude a factory calibration 28 of the swept laser source; obtaininglaser properties 34 of the laser source; or obtaining a parametric modelof the swept laser source. The factory calibration 28 of the swept lasersource may be obtained from the manufacturer of the swept laser source.Obtaining laser properties 34 of the laser source comprises a real timeexternal sensor to obtain optical and environmental data about the sweptlaser source, such as temperature, position of optical elements,gradient, etc. The characterization data 26 may include prior knowledgeabout the swept laser source in the form of coefficients, a look-uptable, or the parametric model to generate the clock signal. A lookuptable (LUT) is a data structure, usually an array or associative array,used to replace a runtime computation with a simpler array indexingoperation. The speed gain can be significant, since retrieving a valuefrom memory is often faster than undergoing an expensive computation orby giving an output value for each of a range of index values. Theparametric model of the laser source can be relied on to generate theswept laser source characterization data. The swept laser sourcecharacterization data may be obtained from the parametric model and areal-time measurement of one or more properties of the swept lasersource. A parametric model is a set of related mathematical equations inwhich alternative scenarios are defined by changing the assumed valuesof a set of fixed coefficients (i.e. parameters). The parametric modelis specified by a functional relationship between model parameters,where some of the parameters can be measured in real time and otherparameters are fixed or factory values. By imputing the model parametersinto the parametric model, the swept laser source characterization datamay be generated. The parametric model can be provided with a softwareprogram in a host PC 58 to create a digital and then analogrepresentation, as shown in FIG. 3. The analog representation willrequire a D/A converter or waveform generator, as described previously.All such characterization data 26 is outputted to the arbitrary waveformgenerator to give a Uniform Frequency Sample Clock signal for thedigitizer.

Pathway 2: Auxiliary Wavemeter Coupled with a A/D and D/A Converter

In one embodiment, the Uniform Frequency Sample Clock 10 includesPathway 2 comprising coupling the swept source 12 to the auxiliarywavemeter 16 and the A/D converter or digitizer 18, as shown in FIG. 4A.The A/D converter 18 is an electronic internal circuit that convertscontinuous analog signals to discrete digital numbers. The D/Aconverter, otherwise known as the arbitrary waveform generator 14, isthen used to output a digitally-processed Uniform Frequency Sample Clocksignal 30 to the external clock signal 34 input of the digitizer 32. TheUniform Frequency Sample Clock signal 30 is repeatedly outputted foreach subsequent optical trigger 54 that occurs as the laser is sweepingthe optical trigger is generated. The optical trigger 54 may begenerated by any of the previously discussed methods.

For one embodiment of Pathway 2, a portion of the light emitted from theswept source 12 is coupled to the auxiliary wavemeter 16 and the opticaltrigger generator 60 via a 50/50 coupler 66 or an arbitrary splittingcoupler, which splits the light into the auxiliary wavemeter 16 and theoptical trigger generator 60. The auxiliary wavemeter 16 may be any typeof wavemeter, including, but not limited to, a Mach-Zehnder, Michelson,or a Fabry-Perot interferometer. Fabry-Perot interferometers arepreferred if the OCT interferometer 40 is phase-sensitive. If the OCTinterferometer 40 system is not phase-sensitive, then Mach-Zehnder,Michelson interferometers, or etalons may be used as the auxiliarywavemeter 16. As shown in FIG. 4B, in one embodiment of Pathway 2, theoptical trigger generator 60 includes a photoreceiver 62 and an inverter64 to generate an electronic trigger signal 54, based onTransistor-Transistor Logic (“TTL”). TTL digital circuits are built frombipolar junction transistors, and resistors with both the logic gatingfunction (e.g., AND, Inversion, etc.) and the amplifying function areperformed by transistors. The optical trigger generator 60 generates theelectronic trigger signal 54 according to when the swept source laser 12light is being emitted. The trigger signal 54 is used to synchronize thedigitizer 32 and arbitrary waveform generator 14 electronics when thelaser has begun a sweep of its light emission. In another embodiment ofPathway 2, the optical trigger generator 60 may be derived from thetuning element in the swept laser source, either the transducer drivingthe tuning element or some transducer reading the tuning element (e.g.,encoder or interferometric signal), which might be light based.Alternatively, the optical trigger generator 60 may be derived bysampling the light emitted from the swept laser source, where thesampling element can be one or more combinations of optical frequencyselective elements, as discussed previously. Combinations of theseapproaches can be employed for the optical trigger generator 60.

As shown in FIG. 4C, in one embodiment, the auxiliary wavemeter 16 is aMach-Zehnder interferometer, where the input from the coupler 66 extendsinto a 50/50 coupler 70 to separate the Mach-Zehnder into two outputpaths. A first output path from coupler 66 extends into a Variable DelayLine VDL 72, a second output path from the coupler 66 extends to aPolarization Controller 74. The variable delay line 72 system consistsof an input fiber, a retro-reflecting mirror on a translation stage, andan output fiber. A manual dial or electrical motor controls the variablelength, or delay, inserted into the optical path, as selected accordingto various factors of the swept laser source being used. The pathlengthdelay determines the clock frequency. Both the polarization controller74 and the VDL 72 extend to a 50/50 coupler 76, which recombines theseparate paths of the Mach-Zehnder interferometer to dual-balancedphotoreceiver 78.

In one embodiment of Pathway 2, the Uniform Frequency Sample Clock 10generates an external sample clock signal 34 linked to the high-speeddigitizer card 32, as shown in FIG. 5. The high-speed digitizer card 32is coupled to the output of the OCT interferometer 40, the output of theauxiliary wavemeter 16, the trigger signal 54 from the trigger generator60, and the arbitrary waveform generator 50. The high-speed PCIdigitizer card 32 can be a dual-channel high resolution 16 bit, 125 MS/swaveform for the PCI bus. The external sample clock signal 34 is derivedfrom an auxiliary optical wavemeter photoreceiver 78 during a start-upcalibration step, and then repeatedly outputted by the arbitrarywaveform generator 50 for each subsequent optical trigger signal 54 thatoccurs as the laser is sweeping. The external clocking system of Pathway2 allows for the wavemeter-generated clock signal to be filtered andprocessed in software before being outputted by the arbitrary waveformgenerator 14. Thus, the external clock derived from the auxiliarywavemeter 16 is regenerated by the arbitrary waveform generator 50 (GageCompuGen) to allow acquisition of data directly in wavenumber (k) space.

In another embodiment of Pathway 2, the auxiliary interferometer 16 is aFabry-Perot interferometer, as shown in FIGS. 6A and 6B. FIG. 6A shows atransmission mode Fabry Perot interferometer 80 made from a ferrule 82having an internal diameter the same OD as the optical fiber claddingconnected from the coupler. The transmission mode ferrule 82 includes asingle mode (SM) optical fiber 84 with a first interface 86 and a secondinterface 88, where each interface is coated with a metal or dielectricmaterial to achieve a desired reflectively. The optimum reflectivity ateach interface will be chosen to maximize interference fringe visibility(i.e., matched intensity reflected from each interface into detectionpath) as is known in the art of fiber Fabry-Perot interferometers.Reflectivity can be controlled by coating the intermediate fiber segment90 of the SM fiber 84 with a carefully-deposited metal or dielectricsurface on each end during assembly. For example, to calculate theoptimum reflectivity (R1) of the first interface in transmission mode,the following quadratic equation is solved so that the two interferingbeams have the same intensity (and max visibility): R1=(1−R1)², which issolved as: R1=38.1%. Quadruple and higher-order reflections will produceharmonics, with much reduced intensity, in the fringe signal, which canprocessed electronically with a filter. Control of temperature of thefiber Fabry-Perot interferometer's thermal expansion/contraction tomaintain a stable path-length difference provides a control for variablefrequency wavemeter output.

The intermediate portion of the fiber segment 90 lies in between thefirst and second interface of the transmission mode ferrule. Thebirefringence in the intermediate fiber portion must be kept to aminimum so that both reflections will have the same polarization state.The optical path length is chosen based on desired interference fringefrequency, which can be 4 mm-6 mm for most OCT swept sources. Thedouble-pass path length determines the clock frequency, as opposed tothe single pass pathlength. The pathlength can be longer, for example insome implementations the optical path length is 10 mm, which can vary todifferent optical path lengths, from 5-20 mm. The pathlength delay inthe Fabry-Perot determines the clock frequency as in other clockinginterferometers.

The light 92 from swept laser source enters through input fiber 84 ofthe ferrule 82. Some light is partially reflected at the first interface86 within the intermediate fiber segment 90 and then discarded; possiblyneeding an optical isolator to protect the source. The remaining lightis transmitted through intermediate fiber segment 90 and partiallyreflected at second interface 88. The reflected portion is transmittedback to first interface, where the light is again partially reflected.Transmitted light is discarded as previously and reflected portion makesa second forward propagation through intermediate fiber segment 90 andis partially transmitted at second interface 88 into an output orcollection fiber 94. This portion interferes with the portiontransmitted into the output fiber from the second interface on theinitial reflection. Thus the path length delay sets the sampling of theoptical frequency signal. The path length delay between the twotransmitted portions is twice the optical path length of theintermediate fiber segment. And the path length delay sets the samplingof the optical frequency signal. Detection of the interference fringesis accomplished after collection of the light with the output fiber,which is coupled to a photoreceiver and the high speed digitizer as theauxiliary wavemeter 16 signal, as previously indicated, to directlyclock the swept source or resample the wavemeter 16 signal in apost-acquisition step.

In another embodiment of the auxiliary interferometer 16, as shown inFIG. 6B, the reflection mode Fabry Perot interferometer 80 includes theferrule 82 having an internal diameter the same OD as the optical fibercladding connected from the coupler. The reflection mode ferrule 82 iscoupled to a polarization-insensitive circulator (not shown) and asingle mode optical fiber 84 with a first interface 86 and a secondinterface 88, where each interface is coated with a metal or dielectricmaterial to achieve appropriate reflectively, as indicated previously.The SM fiber 84 includes an end portion of the fiber segment 90 thatlies in between the first and second interface 86 and 88. The opticalpath length is chosen based on desired interference fringe frequency,which can be about 2 to about 1000 mm for most OCT swept sources. Light92 from swept laser source enters port 1 of a polarization-insensitiveoptical circulator and is send outward on port 2. The fiber comprisingport 2 becomes the input fiber to the inline delay device. Light 92 ispartially reflected at first interface 86 within optical fiber 84. Then,the transmitted portion forward-propagates to the second interface 88,which has a maximum reflectivity. Light reflected from the secondinterface 88 backward-propagates to the first interface 86, where thelight is partially reflected again as a second partially reflectedlight. The transmitted portion from the second partially reflected lightbeam and the reflected portion from the original incident beam theninterfere and are collected on port 3 on the circulator where they arethen detected by a photoreceiver, which is coupled to the high speeddigitizer as the auxiliary wavemeter signal, as previously indicated.

An etalon is not substantially different from a Fabry-Perot, as the twoterms are used interchangeably in the art as Fabry-Perot etalon. TheFabry-Perot etalon can be a fiber version or a free-space version. Inthe etalon approach, incident light (free-space) is multiply-internallyreflected in a highly-controlled and wavelength-specific manner suchthat internal interference allows transmission of wavelengths in aperiodic fashion. Frequency of this periodic transmission functiondepends on the thickness of the etalon and the laser sweep speed(cm^(−l)/s or Hz/s). Adverse environmental effects are reduced by havinglight propagated along a common path, and high finesse provided bycareful control of the facet reflectivity is required.

In another embodiment, Pathway 2 comprises coupling the swept lasersource 12 to the auxiliary wavemeter 16 and a sweep detector 68, asshown in FIG. 7. As the swept laser source 12 starts its sweep, thesweep detector 68 collects an intensity profile of the laser sweep,which is fed into a Delay Generator 98. The Delay Generator 98 has acomparator built in so that it can shape the intensity profile of thelaser into a square wave. This square wave can be varied in length from20-50 uS, depending on what is needed. This 20-50 uS wavelength isdependent on the laser can be easily modified to for different lasersweep speeds and duty cycles. This shaped square wave is then fed into aswitching circuit 46.

The OCT Interferometer 40 operates normally; sending the OCT fringesignal data to the digitizer 32 that is clocked by the process thatoccurs with the auxiliary wavemeter 16. The auxiliary wavemeter 16 canbe any of the previously described wavemeters, Mach-Zehnder, Michelson,Fabry-Perot, inline Fabry-Perot, and the like. The auxiliary wavemeter16 creates an interference pattern that has a non-uniform frequency intime, based on the path length mismatch when the laser is operating.This auxiliary wavemeter 16 signal is converted to an electrical signalusing a balanced photodetector, which is then passed into a circuit thatincludes a buffer 42 and a high pass filter 44. The signal is bufferedfor impedance matching purposes, and then the signal is directed intothe high pass filter 44 to remove low frequency components than a cutofffrequency. The filtered signal is then directed into an electronicswitch circuit 46. The electronic circuit switch establishes connectionsbetween links, on demand and as available, in order to establish anend-to-end circuit between devices. The connections are temporary,continuous, and exclusive in nature. When the laser power is less thanthe threshold level or the wavelength of the laser sweep is outside of acertain range (i.e. a false condition), the trigger signal from sectionis 0 volts, and 5V when the condition is true. The output of the switchcircuit 46 is an external clock when a 0V signal into the trigger of theswitch circuit 46. This external clock is not tied to the laser in anyway and is always running. The output of the switch circuit 46 is thefiltered signal from the auxiliary wavemeter 16 when the trigger voltageis 5V (when the condition is true). This satisfies the condition of someA/D cards to always have a clock on the input, even while the laser isnot on.

Regardless of the output of the switch circuit 46, the signal is highpass filtered through the high pass filter 48. The signal from the highpass filter 48 is coupled into a high speed comparator 38. A comparatoris a device which compares two voltages or currents and switches itsoutput to indicate which is larger. The high speed comparator 38converts the signal from the high pass filter into a square wave thathas a voltage level compatible with the digitizer 32 external clockinput parameters. The high pass filter 48 and comparator 38 help cleanup the signal. The signal is then fed into the external clock on thedigitizer 32 A/D card.

OCT Interferometer

In one embodiment, as shown in FIG. 8, the OCT interferometer 40 cancomprise, a Mach-Zehnder interferometer configuration 100, whichmeasures the complex mutual coherence function (magnitude and phase)between two non-reciprocal optical paths, one path encompassing anobject under test, i.e. “the sample”, and the other a reference path.Alternatively, the OCT interferometer can comprise a Michelsoninterferometer configuration which measures the same coherence functionin a reciprocal configuration, i.e. the same splitter/coupler is usedfor both input splitting and output recombination. A SS-OCT system andcalculations for the OCT interferometer is generally described andexplained by the inventors in U.S. patent application Ser. No.11/446,683, and Provisional Application Ser. No. 60/932,546, hereinincorporated by reference.

The OCT system 100 has swept light source 12 with cascaded fiber opticcouplers to subdivide the source light into three primary modules (1)the primary OCT interferometer, (2) the auxiliary wavemeterinterferometer 16, and (3) the optical trigger generator 60. In oneembodiment, the swept light source 12 is a High Speed Scanning LaserHSL-2000 (Santec) with an instantaneous coherence length of over 10 mm,an 110 nm Wavelength Scan Range, and a scan rate of 20 kHz. Line-arrowsgenerally designate optical fibers coupled the elements of the OCTsystem 100.

As shown in FIG. 8, in one embodiment of the OCT interferometer 100, 90%of the radiant output of the swept light source 12 is split into theprimary OCT interferometer by coupler 110. Coupler 110 splits light intoa coupler 112 and a coupler 114. Then coupler 112 splits light 90% ofthe directed light to port 1 of a 3-port polarization insensitiveoptical circulator 120 for the sample path and 10% of the light isdirected to port 1 of a 3-port polarization insensitive opticalcirculator 122 for the reference path. Port 2 of circulator 120 for thesample path is coupled to a sample 124. The sample path can be coupledto a probe or catheter 126 via a fiber optic rotary junction (notshown). Examples of a rotating catheter tip for the sample path include,a catheter for in-vivo imaging as described in U.S. ProvisionalApplication No. 60/949,511, filed Jul. 12, 2007, a turbine-type catheteras described in Patent Cooperation Treaty application PCT/US04/12773filed Apr. 23, 2004; or a rotating optical catheter tip as described inU.S. patent application Ser. No. 11/551,684; or a rotating catheterprobe as described in U.S. patent application Ser. No. 11/551,684; eachherein incorporated by reference for the methods, apparatuses andsystems taught therein. The catheter can be located within a subject toallow light reflection off of subject tissues to obtain opticalmeasurements, perform medical diagnosis, complete treatment, and thelike.

Continuing with FIG. 8, port 2 of the optical circulator 122 is coupledto a polarization controller 130 and a Variable Delay Line (“VDL”) 132for the reference path. The VDL 132 extends to reference reflector 134.The variable delay line 132 system consists of an input fiber, aretro-reflecting mirror on a translation stage, and an output fiber. Adial controls the variable length, or delay, inserted into the opticalreference path. The typical length variation is about 6 cm, while thetypical time delay is about 300 picoseconds. The VDL 132 provides forlarger path-length adjustments with micron-size adjustment being thesmallest increments.

For the reference path, port 3 of the optical circulator 122 is thencoupled to a 50/50 coupler 116, while port 3 of the optical circulator120 is coupled to the coupler 116 for the sample path. The reference andsample paths encompass the total optical path beginning at the split incoupler 112, into ports 1 of the circulators 122 and 120, out of andback into ports 2 of the circulators 122 and 120, out of ports 3 of thecirculators 122 and 120, and ending at their combination in coupler 116.The coupler 116 includes outputs 3 and 4 to a dual-balancedphotoreceiver 140. The photoreceiver 140 comprise a detection element,such as an InGaAs photodiode and a transimpedance amplifier, whichconverts the electrical current signal generated by photons absorbed bythe photodetector element into a voltage signal that can be read by thedigitizer. Typically, some gain amplification is given at this stage orin a following stage, as well as some filtering for removing noise thatis outside of the relevant electrical signal bandwidth. The gained andfiltered voltage signal is digitized. The OCT interferogram [S(k)] isdigitized at 16-bit resolution using a high-speed PCI digitizer 142board (AlazarTech ATS660, Toronto, Canada) coupled to the photoreceiver140 from the primary OCT signal and the photoreceiver from auxiliarywavemeter 16. The external clock derived from the wavemeter andregenerated by the arbitrary waveform generator (Gage CompuGen) allowsacquisition of OCT signal data directly in wavenumber (k) space. S(k) isconverted using the Fast Fourier Transform (FFT) into the pathlength (z)domain. The magnitude of the transformed OCT A-scan [|S(z)|] representsthe backscattered magnitude at depth z in the sample. The digitizer iscoupled to a computer processor 144, which is a state-of-the-artworkstation with a fast multi-core processor, RAID striped disk array,and large RAM space. Alternatively, the computer processor 144 includesa distributed acquisition and processing system, as described in U.S.patent application Ser. No. 11/868,334, filed Oct. 5, 2007, hereinincorporated by reference.

OCT Depth Calibration and Automated Range Adjustment

Circular and cylindrical OCT scanning devices, i.e. the rotationcatheter scanning devices discussed previously, sample physical space inan inherently polar coordinate system (e.g. radius and angle rather thanlength and width). Circular and cylindrical OCT scanning devices areapplied to image physiological structures with cylindrical-like crosssections e.g., airways and blood vessel lumens). However, digitalrepresentations of the images (i.e. arrays of pixels representingnumeric values) are inherently rectangular. A method for detecting andusing OCT image features, either intentionally or artifactuallygenerated, comprises automatically adjusting the depth range in polar(“radar-like”) OCT images.

Polar OCT images must be converted from their rectangular representationbefore displaying to the viewer. Additionally, if quantitative values(e.g. lumen diameters, lumen areas, circumferences, etc.) are to bemeasured on the polar image, then the transformation from rectangular topolar must preserve relative distances between pixels in all dimensions(radial and angular). Generally, the OCT depth scan (y axis inrectangular coordinates) maps directly to radius and the OCTcircumferential scan (x axis in rectangular coordinates) maps to someincrement of 2*Pi radians (or 360°) polar angle.

For example: y=0 (the top row of the rectangular image) maps to radius=0(the center of the polar image) and y=y_(max) (the bottom row of therectangular image) maps to radius=y_(max) (the perimeter of the polarimage). Likewise, x=0 (the left column in the rectangular image) maps toangle=0° and x=x_(max)/2 maps to approximately 180° and x=x_(max) mapsto an angle of approximately 359°.

For accurate quantitative dimensional measurement in polar images,pixels mapping to radius=0 must represent the actual physical space atthe center of the axis of rotation of the imaging probe, otherwise thepolar image will be artificially warped (expanded or contracted) in theradial direction. However, in an arbitrary OCT image, the pixels at y=0do not necessarily satisfy this requirement and must be shifted in the ydimension until this is satisfied before mapping to a polarrepresentation. Differential displacements (either controlled oruncontrolled) in the path length of the sample vs. reference arms of theinterferometer will shift the pixels in the y dimension.

Uncontrollable displacements can occur when using cylindrical (actuallyhelical)-scanning fiber-optic OCT catheters; for example, when thecatheter is pushed or pulled longitudinally, the fiber-optic cable canbe compressed or stretched and thus a path length displacement isincurred.

The method is an automatic recognition of the uncontrolled displacementeffect based on searching for image features that should be stationary(but are not due to uncontrollable displacement), and successivecalibration of OCT image data so that polar representations can then beused for accurate dimensional measurements. Finally, a method isprovided for subsequent removal of image features in image prior todisplay.

Image features used by the method are generated within the catheteritself (not within the imaged subject or surroundings) and should appearsomewhat stable in depth and consistent in intensity throughout the 360°rotation of the catheter. These include but are not limited to backreflections at interfaces between optical components (aka “ghost-lines”or “echo artifacts”, these occur along the optical axis of rotatingparts and thus appear as uniform circles in the polar image when nodifferential path length displacement occurs over the course of onecatheter rotation), or reflections from the boundaries of or from withinthe stationary (non-rotating) catheter sheath (if it is circular incross-sectional profile and also mechanically concentric with therotating portion).

Steps in the automatic recognition and calibration method could include:(1) Averaging the OCT image frame along the x—(i.e. angular) dimension.This selectively enhances the feature(s) which are rotationally stablein the y dimension (i.e radius) vs. other image features generated bysubject or surroundings. Efficacy of the method is improved if the imagefeature(s) used have high intensity relative to the surrounding pixelsand if subject/environment features (noise) do not have strongcircumferential symmetry; (2) Find feature(s) using peak searching,correlation, thresholding, or other pattern recognition algorithms knownin the art. The efficacy of this method is improved if the range overwhich uncontrolled path length displacements can occur is known apriori, thus limiting the required search space; (3) Compare they-value(s) of feature(s) found in step 2 to a pre-calibrated y-valuewhich represents the actual physical location(s) of that feature(s)relative to the rotational axis, or to the location of a known“conjugate image” or “aliased image” of that feature(s) when usingspectral-domain OCT; (4) Calibrate by shifting the OCT image pixels inthe y dimension by the difference between searched feature(s) andpre-calibrated feature(s). Multiple features can be used to improveefficacy of the algorithm. After shifting the rectangular image in the ydimension, map to polar image coordinates. Radii measured to the centerof the calibrated polar image will represent actual radii measured tothe rotational axis in physical space. Often image features due to thecatheter are unwanted for effective and distraction-free display of thesubject/environment features. For example, the catheter image featurescould overlap the subject/environment features.

Steps to remove (or make less noticeable) the image features couldinclude: (1) Cropping out the image feature(s) extent in the y/radialdirection and in all columns/angles; (2) Calculating the average valueof the pixels immediately inside and outside (above and below) of thecropped region for all columns/angles and inserting this averagedrow/circumference in the cropped location. Unfortunately, the croppingoperation can also remove subject/environment features and distorts theimage in the radial dimension. This distortion makes measurement ofaccurate quantitative values on such images more complicated, becausethe measurement tool must then consider where pixels have and have notbeen cropped (or make the measurement on the un-cropped image).

Pathway 3: Auxiliary Wavemeter Coupled with an Analog Processor

In another embodiment of the Uniform-Frequency Sample Clock 10, Pathway3 comprises the auxiliary wavemeter 16 coupled with an analog processor,shown as in FIG. 1. The auxiliary wavemeter 16 can be any of thepreviously described wavemeters, Mach-Zehnder, Michelson, Fabry-Perot,inline Fabry-Perot, and the like. The analog processor can be anyprocessor (e.g. filtering, pulse shaping, rectifying, and/or switchingprocessor, etc.) that the wavemeter outputs to obtain aUniform-Frequency Sample Clock signal which meets the specifications ofthe digitizer external clock input port. In one embodiment, the analogprocessor is a circuit 200 coupled to the high-speed digitizer to samplethe clock signal, as shown in FIG. 9. During the laser sweep, this clockis the temporally-non-linear but the wavenumber-linear(frequency-linear) wavemeter clock. When the laser sweep is absent, thisclock can be replaced with a dummy clock which has been pre-phase-lockedwith the k-space auxiliary wavemeter clock. Thus, high-speed digitizersare enabled to be operated in a mode where the Uniform-Frequency SampleClock is used to directly sample the OCT signal, which avoids the needto acquire this Uniform-Frequency Sample Clock signal on a differentchannel and post-process data that slows down real time image display.

The sampling circuit 200 for the external sample clock signal is derivedfrom the auxiliary wavemeter 16 during the limited duty cycle of atunable laser source and is derived from a pre-locked (in phase andfrequency) voltage controlled oscillator 270 (“VCO”) during thenon-sweeping segment of each duty cycle, as shown in FIG. 9. A VCO is anelectronic oscillator designed to be controlled in oscillation frequencyby a voltage input. The frequency of oscillation is varied by theapplied DC voltage, while modulating signals may also be fed into theVCO to cause frequency modulation (FM) or phase modulation (PM); a VCOwith digital pulse output may similarly have its repetition rate (FSK,PSK) or pulse width modulated (PWM). A phase locked loop (PLL) is usedto sync the VCO output 274 with the optical wavemeter output before thesweep cycle is complete, at which time the external sample clock isswitched from the optical wavemeter output to the output of the VCO (thedummy clock). A PLL is a control system that generates a signal that hasa fixed relation to the phase of a “reference” signal. The PLL respondsto both the frequency and the phase of the input signals, automaticallyraising or lowering the frequency of a controlled oscillator until it ismatched to the reference in both frequency and phase. When the lasersweep begins again, lock is regained and the output is again switched tothe k-space auxiliary wavemeter output.

The sampling circuit provides a continuous sample clock with acceptablejitter specifications to the digitizer's external sample clock inputport. The locking of dummy and wavemeter clocks in phase and frequencyby the PLL allows a handoff between clock sources to be free fromspurious and instantaneous phase changes and frequency changes whichcould induce an error in the digitizer clock control circuitry.

In one embodiment, the sampling circuit 200 for direct external samplingof swept source OCT data comprises a clock conditioning block 230, aphase comparator 240, a gain-filtering block 244, a voltage controlledoscillator 270, a sample-and-hold block 260, an analog switch 250, adigital switch 290, a timing control block 280, and apost-filtering/deglitching block 294.

As shown in FIG. 9, the clock conditioning block 230 receives an inputfrom the auxiliary wavemeter 16. The clock conditioning block 230 takesa sinusoidal analog voltage generated in the auxiliary wavemeterphotodetector, and the clock conditioning block 230 filters out unwantednoise and DC component using a bandpass filter. The clock conditioningblock 230 generates a digital pulse train (≈0-5V) at same frequency asinput voltage signal and outputs 232 and 234 to the phase comparator 240and main digital switch.

The phase comparator 240 outputs an analog voltage that is proportionalto the difference in phase (and thus frequency) between the signals onits inputs, the conditioned k-space clock 230 and the VCO output 272.The phase comparator 240 can be embodied using various methods such as acharge-pump phase comparator, analog multiplier, an exclusive-NOR logicgate, i.e. an “XOR gate”, etc. The phase comparator 240 outputs to thegain-filtering block 244. The gain-filtering block 244 averages theanalog output voltage from the phase comparator 240 and is used to“tune” the PLL characteristics. The conditioned voltage output from thegain-filtering block 244 controls the VCO 270.

The voltage controlled oscillator 270 outputs a digital pulse train withfrequency proportional to the input 252 voltage from the analog switch250. The pulse train is negatively fed-back into an input 272 of thephase comparator 240. This closed-loop feedback or phase locked loop(PLL) causes the VCO 70 to oscillate in phase with the conditionedk-space clock 230. The phase locked loop syncs the VCO output 274 withthe optical wavemeter output 16 before the sweep cycle is complete, atwhich time the external sample clock 298 is switched from the opticalwavemeter output 16 to the output 274 of the VCO, dummy clock. When thelaser begins to sweep again, lock is regained and the output is againswitched to the k-space auxiliary wavemeter output 16.

The sample-and-hold circuit 260 samples and holds the output 246 voltageof the gain-filtering block 244 shortly before loss of the k-spaceauxiliary wavemeter clock 230. Then the sample-and-hold circuit 260 usesthe analog switch 250 applied to the input 252 of the VCO 270. Thismaintains the VCO 270 output 272, 274 with the same phase and frequencyas it was operating before loss of the k-space auxiliary wavemeter.Sample and hold operation is controlled from a signal 282 in the timingcontrol block 280.

The analog switch 250 changes the input 52 to the VCO 270 between twoanalog sources (1) the sample-and-hold block 260, during dummy clockoperation, and (2) the gain-filtering block 244 during wavemeteroperation. The analog switch 250 is controlled from a signal 284 in thetiming control block. The digital switch 290 changes the output of theentire clocking circuit between the digital conditioned auxiliarywavemeter clock 230 output 232 and the VCO 270 output 274 clock (whensweep is not present). The digital switch is controlled from a signaloutput 286 from the timing control block 280.

The timing control block 280 orchestrates analog switching 284, digitalswitching 286, and sample-and-hold operation 282 based on a triggerinput signal from the swept laser source or other threshold detector.The post-filtering/deglitching block 294 removes any spurious glitchescaused by switching, insures a strong full-range digital signal 296 isavailable for the digitizer external sample clock input.

Pathway 4: Auxiliary Wavemeter Coupled with an Analog Processor and D/AConverter

In another embodiment of the Uniform-Frequency Sample Clock 10, Pathway4 comprises the auxiliary wavemeter 16 coupled with the analog processor20, the AID digitizer 18, and a D/A converter 14, as shown in FIG. 1.Alternatively, a software processing step may be included after the D/Aconverter, or between the A/D digitizer and the D/A converter. Theauxiliary wavemeter 16 can be any of the previously describedwavemeters, Mach-Zehnder, Michelson, Fabry-Perot, inline Fabry-Perot,and the like. The analog processor 20 can be any processor (e.g.filtering, pulse shaping, rectifying, and/or switching processor, etc.)that the wavemeter 16 outputs to obtain a Uniform-Frequency Sample Clocksignal, which meets the specifications of the digitizer external clockinput port. In one embodiment, the analog processor 20 is the circuit200 coupled to the high-speed digitizer to sample the clock signal, asshown in FIG. 9. During the laser sweep, this clock is thetemporally-non-linear but wavenumber-linear (frequency-linear) wavemeterclock. When the laser sweep is absent, this clock can be replaced with adummy clock which has been pre-phase-locked with the k-space auxiliarywavemeter clock. Thus, high-speed digitizers are enabled to be operatedin a mode where the Uniform-Frequency Sample Clock is used to directlysample the OCT data signal, which avoids the need to acquire thisUniform-Frequency Sample Clock signal on a different channel andpost-process data that slows down real time image display.

The analog processor 20 outputs to an AID converter 18, which thenoutputs to the D/A converter. Alternatively, the software processing isincluded after the D/A converter, where the digitized signal isprocessed to a software clock signal that is input to the D/A converter,and then output to the External Clock input on the D/A converter. TheD/A converter 14, is the arbitrary waveform generator, outputs thegenerated Uniform-Frequency Sample Clock signal for each laser sweep,triggered by an electrical synchronization pulse derived from theswept-source laser output. The external clock signal is derived from theanalog processor 20 during the start-up calibration step, and thenrepeatedly outputted by the arbitrary waveform generator 14 for eachsubsequent optical trigger signal that occurs as the laser is sweeping.The Uniform Frequency Sample Clock signal is sent to the digitizer toallow the acquisition of data directly in wavenumber (k) space. From theauxiliary wavemeter, D/A converter, and then the A/D converter, andrepeatedly generating the clock signal, the option of inserting asoftware processing step between the A/D and D/A steps remains.

Pathway 5: Auxiliary Wavemeter Coupled to the Swept-Source

In another embodiment of the Uniform Frequency Sample Clock 10, Pathway5 includes coupling the swept source to the auxiliary wavemeter 16 andto the digitizer 32, without any pre-processing, as shown in FIG. 1. Theauxiliary wavemeter 16 can be any of the previously describedwavemeters, such as the Mach-Zehnder, Michelson, Fabry-Perot, inlineFabry-Perot, and the like. Alternatively, the auxiliary wavemeter 16 maybe a Mach-Zenhder or Michelson interferometer depending on the OCTsystem employed. The auxiliary wavemeter 16 outputs a periodic signaluniformly spaced in wavenumber. The auxiliary wavemeter 16 output isused as an external clock for the High-Speed digitizer so that the OCTsignal date is digitized uniformly in the wavenumber domain [S(k)].Digitizing the OCT signal data uniformly in the wavenumber domain allowsdirect Fourier-transformation into the pathlength (z) domain andconstruction of the OCT image without time-intensive remapping.Following this approach, the nonlinear sweep characteristic of thetunable laser source is effectively removed and OCT images can bedisplayed in real-time.

Pathway 6: Auxiliary Wavemeter and Gas Cell Calibration Coupled to SweptSource

In another embodiment of the Uniform Frequency Sample Clock, Pathway 6includes coupling the swept source 12 to the uniform frequency sampleclock generator 22, as shown in FIG. 1. The uniform frequency sampleclock generator 22 includes an optical wavemeter 300 and a gas cellcalibration 310, as shown in FIG. 10A. A 50/50 coupler 302 splits thelight from the swept source 12 to the optical wavemeter 300 and the gascell calibration 310. The optical wavemeter 300 can be any of thepreviously described wavemeters, such as the Mach-Zehnder, Michelson,Fabry-Perot, inline Fabry-Perot, and the like. Alternatively, theoptical wavemeter 300 may be a Mach-Zenhder or Michelson interferometerdepending on the OCT system employed. The optical wavemeter 300 outputsa periodic signal uniformly spaced in wavenumber. The optical wavemeteroutput 304 is used as an external clock for the High-Speed digitizer sothat the OCT signal data is digitized uniformly in the wavenumber domain[S(k)]. Digitizing the OCT signal data uniformly in the wavenumberdomain allows direct Fourier-transformation into the pathlength (z)domain and construction of the OCT image without time-intensiveremapping. Following this approach, the nonlinear sweep characteristicof the tunable laser source is effectively removed and OCT images can bedisplayed in real-time.

In one embodiment, the optical wavemeter 300 is a fiber-basedFabry-Perot interferometer with a pathlength difference generated by twoin-line partially reflecting surfaces 306 and 308, as shown in FIG. 10B.A single-pass of light 316 and a triple-pass of light 318 between thepartial reflectors 306 and 308 interferes and produces a periodic signaluniformly spaced in wavenumber (k). The pathlength difference isselected to produce a fringe output in wavenumber (k) spacecorresponding to Nyquist sampling of the longest detectable pathlengthdifference. The longest detectable pathlength can be a function ofvarious factors and is always limited by the coherence length of thelaser source. In cardiovascular applications, a fairly long detectablepathlength on the order of 10 min may be applied. With swept lasersources, the pathlength can be as long as a few meters (2000 mm);however, the sweep may be very slow (10 sweeps/s). Sources with a longercoherence length (detectable pathlengths) that have a faster sweepspeed, with a range of 2-2000 mm. Sources with very long coherencelengths can use multiplexing principles, as described in patentapplication entitled “OCT Using Spectrally Resolved Bandwidth, U.S.patent application Ser. No. 11/446,683. The Uniform Frequency SampleClocking pathways are applicable to multiplexed OCT as well.

The partial reflecting surfaces 306 and 308 are encased in amechanically and thermally isolated enclosure to insure phase stability.A 50/50 splitter 312 and parallel balanced detector 314 is incorporatedin the optical wavemeter 300 to reduce noise and improve dynamic range,as shown in FIG. 10B. Harmonics generated by higher order passes betweenthe surfaces are effectively suppressed by the cumulative reflectivitylosses and roll-off due to the finite instantaneous coherence length ofthe laser source (10 mm). The output of the wavemeter 300 iselectrically pre-filtered and amplified into a robust external clock forthe high speed digitizer/analog-to-digital (“A/D”) converter.

As shown in FIG. 10C, the calibration gas cell 310 receives 50% of thelight source from the coupler 302, where 50% of the light is split by a50/50 coupler 320 to a balanced photodetector/amplifier 322. In oneembodiment, the calibration gas cell 310 includes a hydrogen fluoride(“HF”) gas cell 324 (Wavelength References, Mulino, Oreg.) with a 10 mmpathlength and a calibrated absorption fingerprint in the 1250-1350 nmspectral range for the balanced detection scheme. Alternatively, othergas cells can be used as the calibration gas cell 310, with well knownwavelength absorption bands and the pathlength selected according to theswept laser source. The well-known absorption fingerprint bands in theHF gas cell 324 result in a reduced detected intensity in the lighttransmitted through the gas cell 324, and as such provide a metric onthe absolute lasing wavelength at those digitized sampling times. Thesample number or sampling time scale can thus be converted to absolutewavelength at one or more samples, depending on the number of absorptionlines. The detected wavemeter photocurrent signal 328 and the detectedgas cell photocurrent signal 326 are combined in the digitizer toprovide the relationship between the sample number or sampling time andlasing wavelength throughout the entire sweep. The detected photocurrentsignal 326 from the gas cell is digitized concurrently with the OCTsignal data and correlated with the known HF fingerprint to determinethe wavenumber bias (k_(o)) of the swept source laser. Knowledge ofwavenumber bias (k_(o)) allows accurate determination of the absolutewavenumber of each digitized sample throughout the spectral sweep,effectively removing any wavenumber offsets and/or phase instabilitiesin the laser source, wavemeter and sampling electronics.

The uniform-frequency sample clock signal which is based on theauxiliary wavemeter represents uniform intervals in wavenumber (k)biased by an unknown absolute wavenumber (k_(o)). Unfortunately, sincethe wavenumber bias (k_(o)) can vary between successive laser sweeps asa result of inherent instabilities in the tunable laser output spectrum,k_(o) must be measured for each laser sweep for highly sensitive phasemeasurements. Gases with molecular absorption lines at NIST-calibratedwavenumbers provide unmatched stability and are used to calibrateoptical spectra in a variety of high-precision spectroscopyapplications.

Gas Cell Trigger

As shown in FIG. 11, in another embodiment of the uniform frequencysample clock generator 22 includes coupling the laser swept source 12 toa single channel detector 330 and the gas cell 310. The laser sweptsource 12 power from the optical trigger 60 channel is divided into thegas cell channel 332 and a window channel 334 with the use of a coupler(not shown). The photocurrent of light passing through the gas cell 310provides a more repeatable and stable optical trigger. Anintensity-thresholded optical signal can suffer from variations inintensity of the laser while an absorption line in a gas cell does notvary and can provide a highly stable wavelength reference. The gas cellchannel 332 and the window channel 334 propagate light simultaneously.The gas cell channel 332 may include>90% of the total trigger channeloptical power. A coupler 340 is used to split the light into a referencechannel 342 and a gas channel 344. In the gas channel 344, light passesthrough the gas cell 310 and a gas cell pulse 356 is outputted to one ofthe inputs of the balanced detector 346, while the reference light isdirectly outputted to the second input of the detector 346. The outputvoltage of the detector 346, which consists of pulses corresponding tothe gas cell 310 absorption lines, is used as the input of adifferentiator 350. The differentiator 350 is an electronic device wherethe output is the derivative of the input. For example, thedifferentiator may be a high pass filter. By differentiating thebalanced detector 346 output, the maximums of the absorption lines arereplaced with a zero crossing voltage. To produce theTransistor-Transistor Logic (TTL) pulses with rising edges correspondingto the central wavelength of the absorption gas cell 310 lines, a delaygenerator 352 (DG1) is coupled to the output of the differentiator 350.The level of voltage that used for generating the pulses should beseveral fold (by absolute value) above the RMS noise level to exceed thenoise floor level and avoid generating pulses from noise. The timeduration of the pulses should be at least several times less thandistance between neighbor gas cell pulses. The time duration of thewindow pulse should be at least several times less than time betweenneighboring gas cell absorption line pulses to prevent false triggering(during one A-scan window pulse should be always overlapped with theonly selected gas cell pulse).

The window channel 334 may include about 10% of the total power of thetrigger channel. The light in the window channel 334 is detected withthe single channel detector 330, so the shape of the detected voltage isrepeating the shape of the laser sweep. The output of the single channeldetector 330 is coupled to a delay generator 348 (DG2), which is used toproduce a window pulse 354. The window pulse 354 is used to select oneof the gas cell pulses 356 among others. The position during the sweepwhere the window pulse 354 starts is adjusted with the voltage level.The start position and width of the window pulse 354 are chosen so thewindow pulse 354 should totally cover one of the gas cell pulses 356.Since the gas cell pulse 356 is fixed in the wavenumber domain thewindow pulse 354 is uttering in the wavenumber domain from sweep tosweep. Therefore, width of the window pulse 354 should be several timeswider than the selected gas cell pulse 356 width, so that the windowpulse 354 covers the gas cell pulse 356 for every sweep. The windowpulse 354 does not cover any of the neighboring gas cell pulses 356.

The outputs from DG1 352 and DG2 348 are used as input of a logicalelement AND gate 360 or NAND gate 362. The main condition for thelogical element is its output when both inputs are high (logical 1)should be different from any other possible input logical states. Theoutput of the logical element is the single TTL pulse with regulatedwidth which is fixed at a specific wavelength and can be directly usedas a gas cell trigger 364 for acquisition of the OCT signal data.

The gas cell trigger 364 is tightly connected with a referencewavelength, where the source of the reference wavelength is the gas cell310. The gas cell 310 is a hermetic capsule containing a known gas, asdescribe previously. The central wavelength of absorption lines of thegas depend on molecular energy levels and practically do not depend onthe external conditions such as temperature. If the swept laser sourceis centered at 1310 nm, then the gas cell 310 should have appropriatecorresponding centered absorption lines. The need for the trigger 364fixed at selected wavelength is a particular interest for phasesensitive OCT, where phase is determined as

${\varphi = {{{kn}\;\Delta\; z} = \frac{2\pi\; n\;\Delta\; z}{\lambda}}},$where n is the refractive index, Δz is the in pathlength differencebetween the sample and reference arms of OCT interferometer, λ is thewavelength of light. Therefore, to have Δφ=0.1 at Δz=0.2 mm, theuncertainty of wavelength should be <10 pm. The sweep to sweepwavelength dependence of the swept source is several orders greater.Using the Pathway 6, the uncertainty is <2 pm for a 20 kHz scan rate ofthe swept source (with increasing swept source scan repetition rate theuncertainty of the trigger position increases linearly). Phase sensitiveOCT provides additional contrast that may be color coded onto OCTintensity images.

The gas cell trigger may be used for any OCT imaging system. The needfor a trigger depends on the stability (instability) of the lasersource. If the source is highly spectrally stable in time (i.e. thedrive signal of the tuning element) then need for a gas cell trigger isless. Generally, the gas cell can provide a highly stable trigger. Afterthe gas cell trigger 354 has been produced, the trigger 354 may be splitinto two signals. One signal is used to trigger the A/D digitizer card(Alazar) to start acquiring A-scans, and the other signal is directed totrigger the external clock circuitry 370, as shown in FIG. 12 Theexternal clock circuitry 370 comprises a delay generator and ak-space/dummy clock switching circuit D. The delay generator comes firstand uses an edge detect to sense when the differentiation circuit(trigger from the gas cell circuitry, 354) has gone high. After the edgehas been detected, the delay generator outputs a 5V signal 372, wherethe time duration may be fixed using a resistor-capacitor combination.Time duration of the 5V pulse from the delay generator is selected toensure sufficiently high signal to noise ratio of the K-space clock. Inone embodiment, the delay generator can be programmed to provide a pulseduration from (20-50) μs. The 5V signal pulse 372 goes into the clockcircuitry 370.

The clock circuitry 370 is composed of a buffer amplifier, a high passfilter, a switching network that can switch between a k-space 374 and adummy clock 376, another high pass filter, and a comparator thatconverts the sine wave of the k-space 374 to a TTL signal 378. Theresulting clock has a constant step in wavenumber space (k) during the(20-50) is of the pulse duration from the delay generator 380 andconstant duration in other time periods. The OCT data signal is acquireduniformly in wavenumber space 374 provided by the external clockcircuitry 370.

As shown in FIG. 12, the original pulse from the gas cell circuitry 354is used to produce the (20-50) μs pulse from delay generator 380 to beused to generate switching between k-space 374 and dummy clocks 378. TheTTL pulse train 378 is produced from a sinusoidal signal and used as afinal clock at the external clock input of the digitizer (ADC board).

Common Path OCT Interferometer

In one embodiment, the OCT interferometer 40 is a common pathinterferometer 400, as shown in FIG. 13. The common path interferometer400 comprises a Phase-Sensitive Fourier Domain OCT system 400(“PS-FD-OCT”) system wherein reference and sample light propagate in acommon optical path 410. The common optical path 410 can propagate in anoptical fiber, free space or some other material. Any environmentallyinduced perturbations in the common path experience common-moderejection and phase-stable OCT signal data results. Some portion of thecommon optical path needs to be different, that is some portion of thesample path is distinct from the reference path. So while the referenceand sample share some portion of the path, some portion of the samplepath is distinct from the reference path.

As shown in FIG. 13, the optical layout of the common-path OCTinterferometer employs a coupler 412 splitting light to a 3-portpolarization-insensitive optical circulator 414 and a 3-portpolarization-insensitive optical circulator 416. The circulator 414includes a source light input on port 1, common reference and samplepaths on port 2, and the output to a balanced photoreceiver 420 on port3. Light is split (30%) to the circulator 416 from the input channel toa variable reflector 422 to reduce noise and improve detector dynamicrange for the balancing channel 426 of the photoreceiver 420. Thevariable reflector 422 in the balancing channel insures equal powerlevels and spectral shape on the balanced detector's 420 twophotodiodes. The distal end of the common-path fiber is terminated witha focusing gradient-index 424 (GRIN) lens. The GRIN lens 424 isoptimized for <−65 dB return loss to minimize interference from spuriousback-reflections, and may include a working distance of 5 mm and focusedspot size of 20 um. A wedged 50% beam-splitter is aligned in the beam toprovide a reference reflection. The sample may be positioned on twomotorized linear translation stages and actuated in a raster pattern tocreate three-dimensional OCT volume scans. Alternatively, the samplepath can be coupled to a scanning system with a flat and calibratedoptical field. Such scanning systems are known in the art of opticaldesign and can include for example a galvanometer, a scanning lens andfield flattener lens. Alternatively, the sample path can be coupled to aprobe or catheter via a fiber optic rotary junction. Examples of acatheter for in vivo imaging in the sample path include, U.S.Provisional Application No. 60/949,511, filed Jul. 12, 2007, aturbine-type catheter as described in Patent Cooperation Treatyapplication PCT/US04/12773 filed Apr. 23, 2004; or a rotating opticalcatheter tip as described in U.S. patent application Ser. No.11/551,684; or a rotating catheter probe as described in U.S. patentapplication Ser. No. 11/551,684; each herein incorporated by referencefor the methods, apparatuses and systems taught therein. The cathetercan be located within a subject to allow light reflection off of subjecttissues or nanoparticles to obtain optical measurements, medicaldiagnosis, treatment, and the like.

As shown in FIG. 14, the common path OCT interferometer 400 is coupledto a FMDL tunable laser source 430, where the FMDL source 430 is coupledto the optical wavemeter 300 and the calibration gas cell 310. The OCTinterferogram [S(k)] and calibration gas cell signature are digitized at16-bit resolution on two channels of a high-speed PCI digitizer 432board (AlazarTech ATS660, Toronto, Canada). The external clock derivedfrom the wavemeter 300 output and allows acquisition of data directly inwavenumber (k) space. S(k_(o)) is shifted to remove any bias asdetermined by the gas cell 310 absorption fingerprint and convertedusing the Fast Fourier Transform (FFT) into the pathlength (z) domain.The transformed OCT A-scan [S(z)] is a complex signal {|S(z)|,arg[S(z)]} representing the backscattered magnitude and phase at depth zin the sample. The digitizer 432 is coupled to a host PC 434 is astate-of-the-art workstation with a fast multi-core processor, RAIDstriped disk array, and large RAM space. The complex signal representingthe A-scan may be used as input into an algorithm to solve the inverseproblem to estimate the refractive index profile (n(z)) of the sample.

FIG. 15 compares axial point spread functions and OCT images generatedwith both uniform time sampling and the uniform frequency sampleclocking approach 10 using the previously discussed Pathways. The graphshows the OCT point spread functions vs. depth for an internallyclocked/remapped scheme 440 and the novel externally clocked scheme 442;the larger height and narrower width of the externally clocked functionsresults in greater signal-to-noise ratio (“SNR’), improved axialresolution, and suppressed artifacts, especially at larger depths (2.5mm-3.5 mm). Additionally, the externally clocked scheme is lesscomputationally and bandwidth intensive.

Multiple Uniform Frequency Clock Signals

For each acquisition channel, one clock signal may be active at a giventime, which may be switched between different clock signals in anyparticular combination or order. Alternatively, more than one uniformfrequency clock signal may be synchronously coupled to the ADC channelthrough a circuit that combines/alters the two clock signal to produce asynchronous signal that reveals something more than just one clocksignal. As shown in FIG. 18, multiple clock signals from variouspathways is inputted into a demultiplexer 600, where the demultiplexerhas an input (possibly digital) that selects one of the input signals.The demulitiplexer would be applicable for the real time clock signalderived from the wavemeter and a backup clock signal is provided just incase the real-time circuitry fails or is intermittent in one way oranother.

Phase Sensitive OCT System

As shown in FIG. 16, an alternative Phase-Sensitive OCT (PS-OCT) system500 comprising a signal interferometer 510, a reference interferometer520, a clocking interferometer 530, a spectrally fixed trigger 540. Theswept source laser 502 is coupled to an 80/20 splitter 504. The splitter504 is coupled to a splitter 506 (95% transmittance, 5% reflection) anda splitter 508.

Light (λ=1310 nm, Δλ=100 nm, 20 KHz scan rate) emitted from a sweptlaser source 502 (Santec, Hackensack, N.J.) is input into four opticalsubsystems: the signal-interferometer 510; the reference-interferometer520; the clocking-interferometer 530; and the spectrally fixed trigger540. The sample under test 512 is positioned in the signalinterferometer 510. Interference fringes (F_(s)(ν)) are formed betweenlight reflected from a splitter 514 and the sample 512 and directed intoChannel A 552 of an analog-to-digital (A/D) converter 550 (ADC). Theinterference fringes (τ_(r) (ν)) in the reference interferometer 520 areformed analogously to τ_(s)(ν) between light reflected from a splitter524 and a high reflection mirror 522 and directed into Channel B 554 ofthe ADC 550. Interference fringes (τ_(cl)(ν)) in the clockinginterferometer 530 are formed between light going through a first arm532 and a second aim 534 of the Mach-Zehnder clocking interferometer530, and after analogous bandpassing 536 served as a real time externalclock 538 source for the ADC 550. The frequency of the external clock538 depends on the optical path difference between 532 and 534 andvaried with a variable Delay Line (“DL”) 542. A sequence of thenarrowband TTL like pulses are formed after light is outputted from agas cell 544 (Wavelength Reference, Mulino, Oreg.) in the spectrallyfixed trigger subsystem 540. The only pulse is selected using a timewindow produced out off a single channel detector (“SCD”) 546 and servesas the spectrally fixed trigger for the ADC 550 at the AND gate usingTTL pulse produced from laser sweep intensity profile.

Accuracy and sensitivity of the phase sensitive OCT instrument can bemeasured using a set of standardized metal films commonly used forcalibrating resolution of atomic force microscopes. Sensitivity of thephase sensitive Fourier-domain OCT instrument is measured by placing areflecting surface on a piezoelectric stepper (PolyTech PI, Auburn,Mass.) with 0.5 nm resolution. After aligning light reflection from thepiezoelectric stepper, one-hundred A-scans are recorded for eachposition and the stepper is incremented 10 nm. Proceeding in thismanner, accuracy and sensitivity of the phase sensitive Fourier-domainOCT instrument can be calibrated. Optical scanning systems thatincorporate field flatteners can provide optical fields that are flat towithin a fraction of a wave (e.g., 1/10 wave) a calibration proceduremay be employed to correct residual phase variations that occur whenscanning across the field. Calibration procedures using precisionreference optical flats as reflective surfaces may be employed tocorrect for phase variation over the field curvature due to the scanningoptics.

The strain constant of the PVDF copolymer piezofilm is d₃₃=−38*10⁻¹² m/V(Images SI Inc, Staten Island, N.Y.). A 10 V amplitude sinusoidalvoltage to the piezofilm is applied using an Agilent function generator,which corresponds to 20 V peak peak change of voltage. The result changein the thickness of the piezofilm equals d₃₃*20V=0.76 nm. The voltagefrequency was 500 Hz (FIG. 17A), 1000 Hz (FIG. 17B) and 2000 Hz (FIG.17C).

The phase measurement versus time for the piezofilm induced by applied10 V sinusoidal voltage with three different frequencies 500 Hz (17A),1000 Hz (17B), 2000 Hz (17C). The measured phase (Y-scale on the right)is due to changing in thickness of the piezofilm. The thickness of thepiezofilm (the Y-scale on the left) can be calculated from the phasemeasurements as T=φ*λ/(4*π), where φ—measured phase (rad), λ—centralwavelength of the Swept Source laser (nm).

K-space Clock Dispersion Correction

The difference in the dispersion between the reference path and thesample path is a common problem in OCT systems. The difference indispersion between the reference and sample path can cause a degradationof the point spread function and reduce image quality. Mathematically,the point spread function S(x) is represented by Equation (1):S(x)=∫F(k(t)e ^(iΔφ(k(t))) e ^(ik(t)x)(dk/dt)dt+c.c.,  (1)where S(x) is point spread function of the swept source OCT system fromone sharp boundary; F(k(t)) is the power spectrum of the swept sourcelaser; Re(e^(iΔφ(k(t)))))=D, is the dispersion mismatch component in thearms of OCT interferometer; dk/dt=C is the component due to non-uniformclocking in k-space; k is the wavenumber; t is time; and c.c. is thecomplex conjugate. Dispersion D can be altered, so D*C=1.

One method addressing the degradation of the point spread function is toaccount for the dispersion and apply a complex resampling algorithm tothe raw data acquired from the Analog to Digital Converter (ADC).Another method comprises dispersion matching of the sample path to thek-space clock path with hardware dispersion matching. The k-space clockpathway is discussed previously. A hardware based approach comprisesdispersion correcting the k-space clock to include the difference indispersion between the sample and reference path. After the differencein dispersion has been measured, it is used to modify the k-space clock.If the OCT system is being clocked by an arbitrary waveform generator, anon-dispersion correct k-space clock signal is acquired by the ADC, thenon-dispersion correct k-space clock signal is modified to take intoaccount the difference in dispersion, and then this dispersion correctedk-space clock signal is used to clock the ADC to acquire the OCT data.

Alternatively, the method comprises modifying the k-space clock opticsto account for the dispersion mismatch in the sample and reference path.The k-space clock optics includes some kind of interferometer that isrelying on interference. The k-space clock corrective optical elementscould include a glass window, fiber optic elements, a plurality ofprisms, and/or a plurality of air gaps. By modifying the optics of thek-space clock then the effect of dispersion mismatch between referenceand sample paths in the interferometer may be corrected. The dispersioncorrected k-space clock optics enables the OCT system to be clocked withminimal dispersion in “real time”, since the k-space clock would notrequire non-causal resampling techniques. The real time dispersioncorrected clock could also be coupled with the arbitrary waveformgenerator pathway, where the difference is the real time dispersioncorrect k-space clock does not require dispersion resampling beforebeing used.

The dispersion characteristic D is usually smooth and cosine modulated(D=Cos [Δφ(k(t))]) versus k but C component can vary non-smoothly fromdigitized sample to digitized sample (neighbor k numbers).

The C component can vary from A-scan to A-scan. From the arbitrarywaveform generator 50 (Gage CompuGen), the C component does not changefrom A-scan to A-scan. However, the dispersion component may still needcorrection from A-scan to A-scan in some embodiments of the arbitrarywaveform generator.

Another approach addressing the degradation of the point spread functionis the presence of an amplitude optical filter Φ(k(t)) in the referencearm of interferometer, which converts Equation (1) to:S(x)=∫F(k(t)e ^(iΔφ(k(t))) e ^(ik(t)x)(dk/dt)Φ(k(t)dt+c.c.,  (2).

The amplitude optical filter Φ(k(t)) facilitates to correct thedistortion of the resolution with depth due to non-uniform k-spaceclocking (D*C*Φ=1 may be easier to achieve than D*C=1). The depthresolution of the OCT system can be improved by effectively increasingthe bandwidth of the power spectrum F(k(t)). Some power in the referencearm may be lost by damping central k-numbers and not modifying k-numbersat the edge of the spectrum.

All the Uniform Frequency Sample Clocking 10 Pathways, systems, andmethods provide for external clocking of a swept laser source and canprovide different Pathways, independently or in combination, to generatethe clock, process the clock, and transmit the clock to the digitizerfor uniform sampling of detected light in the wavenumber domain.Alternatively, all the Uniform Frequency Sample Clocking 10 Pathways maybe combined with each other, in any particular combination or order. Forexample, an optical parameter of light can be measured by the clockingsystem and the optical parameter can be used in a model or look-up-tableto predict the clocking wavenumber for a swept-source OCT system.

In one embodiment, the Uniform-Frequency Sample Clock Pathways for OCTsystems image at least a portion of a sample. In one embodiment, thesample is a biological sample. The sample path of the OCTinterferometers can be coupled to a probe or catheter via a fiber opticrotary junction to image a biological sample. The catheter can belocated within a subject to allow light reflection off of subjecttissues or nanoparticles to obtain optical measurements, medicaldiagnosis, treatment, and the like. In one embodiment, theUniform-Frequency Sample Clock Pathways is coupled to OCT systems andcatheters for imaging blood flow, such as in U.S. patent applicationSer. No. 11/550,771, imaging a vessel or internal lumen of a patient,such as in U.S. patent application Ser. No. 11/446,683, and imagingnanoparticle labeled cells, such as in U.S. patent application Ser. No.11/441,824, each herein incorporated by reference for the methods,apparatuses and systems taught therein.

While the embodiments have been described, it will be understood thatthe invention is capable of further modifications. This application isintended to cover any variations, uses or adaptations of the inventionfollowing, in general, the principles of the invention, and includingsuch departures from the present disclosure as, within the known andcustomary practice within the art to which the invention pertains.

What is claimed is:
 1. A method for generating an optical coherencetomography (OCT) image, the method comprising: producing light from aswept laser source; splitting the light into a first and a secondportion, wherein the first portion is sent to an OCT interferometer toproduce an OCT signal and the second portion is sent to a uniformfrequency sample clock; processing the second portion into an externalclock signal that is accepted by a digitizer, wherein processingcomprises the steps of coupling the swept source laser to a uniformfrequency sample clock generator and generating the clocking signal,wherein the uniform frequency sample clock generator further comprises agas cell providing a metric on the absolute lasing wavelength atdigitized sampling times and an optical wavemeter coupled to a digitizerproviding the relationship between the sampling time and lasingwavelength, further comprising determining the wavenumber bias of theswept laser source; sending the OCT signal and the clock signal to thedigitizer; and generating an OCT image from a combination of the OCTsignal and the clock signal.
 2. A method for generating an opticalcoherence tomography (OCT) image, the method comprising: producing lightfrom a swept laser source; splitting the light into a first and a secondportion, wherein the first portion is sent to an OCT interferometer toproduce an OCT signal and the second portion is sent to a uniformfrequency sample clock; processing the second portion into an externalclock signal that is accepted by a digitizer, wherein processingcomprises the steps of coupling the swept source laser to a uniformfrequency sample clock generator and generating the clocking signal,wherein the uniform frequency sample clock generator further comprises adetector channel and a gas cell channel to generate a gas cell pulse,differentiating the gas cell pulse to replace the maximum absorption gascell lines with a zero crossing voltage, producing atransistor-transistor logic pulse with rising edges corresponding to thecentral wavelength of the absorption gas cell lines, repeatedlyoutputting the shape of the laser source sweep, and producing a windowpulse to select one of the gas cell pulses; sending the OCT signal andthe clock signal to the digitizer; and generating an OCT image from acombination of the OCT signal and the clock signal.
 3. The methodaccording to either claim 1 or 2, wherein the OCT interferometercomprises a Mach-Zehnder interferometer configuration.
 4. The methodaccording to either claim 1 or 2, wherein the OCT interferometercomprises a Michelson interferometer configuration.
 5. The methodaccording to either claim 1 or 2, wherein splitting is accomplishedusing optical couplers.
 6. The method according to claim 5, wherein 90%of the first portion comprises 90% of the light.
 7. The method accordingto either claim 1 or 2, wherein the OCT interferometer is coupled to acatheter probe.
 8. The method according to either claim 7, wherein thecatheter probe is a rotating catheter probe.